Recently, a variety of optical recording media have been developed and an optical pick-up device that may be shared by multiple types of optical recording media in order to record and reproduce signals have been manufactured. For example, it is known in the prior art to use a single optical pick-up device with either a DVD (Digital Versatile Disk) or a CD (Compact Disk including CD-ROM, CD-R, CD-RW) in order to record and reproduce signals. For these two optical recording media, the DVD uses visible light having a wavelength of approximately 657 nm for improved recording densities while the CD is required to use infrared light having a wavelength of approximately 790 nm because some recording media are insensitive to visible light. The optical pick-up device shared by these two recording media uses illumination light of two different peak wavelengths.
The two optical recording media described above require different numerical apertures due to their different features. For example, the DVD is standardized to use a numerical aperture of 0.6 and the CD is standardized to use a numerical aperture in the range of 0.45-0.52. In prior art devices different numerical apertures are used depending on the optical recording media, and all aperture diaphragm, such as a liquid crystal shutter or a wavelength selective filter, may be used to achieve the different numerical apertures. Alternatively, multiple diaphragms may be interposed to achieve the different numerical apertures.
However, the prior art techniques as described above increase the size of the device, as well as increase its complexity and cost.
The inventors of the present application has previously disclosed in Japanese Patent Application 2002-156854 an objective lens that has a zonal part on one of lens surface at the outermost periphery thereof and that has a certain depth (or height) so as to apparently eliminate the light of one of tile wavelengths at the flux periphery and while maintaining the light of the outer wavelength. This objective lens eliminates the need for diaphragms, as provided in the prior art, and results in a downsized optical pickup device that can be produced at a reduced cost.
The objective lens as described in Japanese Patent Application 2002-156854 has a circumferential stepped part at the boundary between the outermost peripheral area and an area inside thereof on one surface (with a depth, for example, equal to (2n+1)λ/2) for one of the wavelengths λ, with n being an integer) so as to form a zonal part on one of the surfaces of the objective lens at the outermost peripheral area and thereby substantially reduce the numerical aperture at the periphery for one of the wavelengths. However, this structure limits the position of the stepped part to a point corresponding to a difference in numerical apertures between two optical recording media (i.e., to at a certain distance from the optical axis). In other words, in order to apparently eliminate the intensity of light at the periphery due to interference effects, the stepped part should be provided nearly at the center of the region that contributes light to cause the destructive interference.
The structure above does not give freedom of design with regard to the position at which the stepped part is formed. Therefore, it is difficult to design a lens having improved optical performance. Consequently, a beam profile corresponding to a required numerical aperture may in some cases not be obtainable. Prior art optical recording media objective lenses having three zonal parts are described for example in Japanese Laid-Open Patent Applications H09-145994, H09-145995, and H09-197108. However, the zonal parts are not for producing an interference effect and the two outer zonal parts converge light fluxes having different wavelengths from each other onto different predetermined points. Thus, the basic technical concepts differ from the present invention.